how can I get xbox live to connect to my mac?

I have an xbox 360. I also have a mac running os X (10.5.8). I am not having any luck getting xbox live to connect to my mac. I run the Xbox test. The xbox sees my wireless network but then this message appears
"Can't obtain ip address from your router or modem"

I have turned my Dlink on and off and my computer. but no luck. I called xbox and they said they couldn't help b/c it was a problem with the setting on the mac. so, i am turning to you folks for advice.